Why are carp a problem in Australia?
First introduced in Australia in 1859, carp became a major pest in the 1960s after the accidental release of a strain that had been adapted for fish farming. Within a few years they established themselves throughout the entire Murray-Darling Basin. Carp now comprise up to 90% of the fish biomass in parts of the Basin. This is largely attributed to female carp producing up to a million eggs per year, and to the omnivorous fish’s tolerance for a wide range of habitats including degraded water.

While we may not be able to ‘prove’ that carp directly caused the degradation of our rivers, their dominance must certainly contribute to the problem. It is unlikely that the Murray-Darling Basin could ever return to its previous glory while carp remain in such high numbers.

What is the carp herpesvirus?
The carp herpesvirus (called Cyprinid herpesvirus 3) is highly specific to carp, including the ‘koi’ ornamental variety, and only causes death in carp, with no other fish known to be affected, even the closely related goldfish. The carp herpesvirus is a water-borne virus and is highly contagious. Viral particles in water may be active for up to three days. New outbreaks of the disease can be expected when fish are stressed or in large aggregations. Clinical signs of disease, including mortality, are most common when water temperatures are 18°–28°C. There is little, or no disease above 30°C or below 15°C.

How does the carp herpesvirus work?
The carp herpesvirus mainly damages the kidneys, skin and gills of koi carp. Kidney and skin are very important in helping the fish maintain its water balance. Animals living in a freshwater environment need to stop water getting into the body (skin) and to pump out excess water that does get in (kidneys). In affected fish there is a water imbalance which causes a mineral imbalance.

Among other things, the latter could affect heart function (although that has not yet been shown in carp). Damage to the gills affects the carp’s ability to breathe and this is the cause of death. After a fish is infected by the virus, the virus multiplies in the fish for about seven days (depending on the water temperature). During this time the fish eats and appears quite normal. It then takes about 24 hours from the first signs of disease (darkening of the skin; reddened gills) until the fish dies.

What are the effects of the carp virus on other species of fish?
Eight years of testing at the CSIROs Australian Animal Health Laboratory (AAHL) has proven that the carp herpesvirus does kill carp found in Australia, and it kills them fast. The virus only replicates in Common carp, and so will not cause disease in other species. This is not surprising, as herpesviruses are generally specific to a single host species, and the research confirms this.

Specifically, CSIRO testing has shown that it poses no danger to 13 native species such as Murray cod, various species of perch, eel and catfish, as well as a crustacean (yabbies) and a non-native fish species, the rainbow trout. The studies have shown that there are no clinical or pathological changes in these non-target animals, nor is there any evidence that the virus multiplies in these species.

How long will it take for the virus to start affecting carp?
Under the right conditions a fish can become infected with the virus and swim around with no clinical symptoms up to 2-7 days, once clinical signs appear, they can die within 24 hours. So it is quite rapid. Based on lessons learnt from past use of viral biocontrol agents for invasive vertebrates, we expect that CyHV-3 will have the greatest impact in the first couple of years after release. After that, its effectiveness may be diminished, but not lost, as virus and host adapt to each other.

How will the virus be rolled out?
Considerable planning is required before a release of carp herpesvirus will occur, including further research into how to manage the release and clean up and ecological impacts, as well as legislative approvals and community consultation. The newly funded National Carp Control Plan will set out a considered approach to safely manage this very significant work. In terms of its nature and scale, this proposal is unprecedented in Australia, and we will need to work across portfolios and governments, in partnership with communities, to ensure that we get it right.

A ministerial taskforce, led by the Minister for Agriculture and Water Resources, will be established to bring together Australian Government ministers from the Environment, Industry, Innovation and Science and Infrastructure and Transport portfolios to oversee the development of the plan. The NSW Minister for Primary Industries will also join the taskforce, given the leading role that NSW continues to play in the technical and regulatory aspects of the carp herpesvirus proposal, and to represent state and territory governments.

A National Coordinator will work with stakeholders from across governments, industry, community and environmental groups, and research organisations to understand the issues and bring together a comprehensive plan, underpinned by research, risk assessment and a sound understanding of community views. The plan will focus on maximising the reduction of carp populations while minimising disruption to industries, communities and the environment should a carp herpesvirus release go ahead.

How will the virus affect the environment?
It is vitally important to ensure that we protect water quality to ensure ongoing access to clean water for human use. This will be managed by using appropriate methods to effectively remove dead carp from the waterways. Detailed research and modelling is currently being undertaken in collaboration with researchers from Water NSW to inform planning for the clean-up strategy. This work will identify carp biomass thresholds that impact on water quality, which can then be used to work out how much carp needs to be removed from the system to prevent negative impacts. International case studies from places like Japan and North America where large-scale clean-up efforts have been successfully employed have also been investigated to help with formulating our approach.

Will the carp herpesvirus affect humans?
The carp herpesvirus has devastated carp farming around the world yet despite the large numbers of people working on these affected farms, there has been no evidence of any effect of the virus on them.

Chickens, mice, frogs, turtles and water dragons have also been tested as representatives of a wider community of birds, mammals, amphibians and reptiles. Again the virus has shown no effect on them which also makes us confident that it won’t affect that other major group of mammals – humans.

A report to the European Commission by the Scientific Committee on Animal Health and Animal Welfare stated that there is no evidence for ANY fish virus causing disease in humans. Researchers have also tried to culture the carp herpesvirus on human cell lines and cell lines of other primates without success.

Fish farmers in many countries around the world including Israel and the United Kingdom have been in close contact with the carp herpesvirus on a daily basis now for decades, without a single documented human health concern.

How will the Carp be cleaned up?
If an image of rivers full of large dead carp floating on the surface is what springs to mind, rest assured that our research includes careful planning and modelling before release and follow-up strategies are recommended. Carp breed in well-mapped specific sites along the Murray-Darling Basin. The virus is likely to be released in these sites where most carp are juveniles. Not only would this wipe out large populations of carp before they become mature, but bird life will probably clean up large numbers of the immature carp.

Study tours of Japan and Indonesia are part of the Invasive Animals-CRC program to study natural outbreaks of carp herpes virus. Researchers will be reviewing the significance of dead mature fish, and strategies for dealing with them.

Could carp develop an immunity to the virus?
Whenever a virus is used as a biocontrol agent, the virus kills large numbers of the target for the first couple of years. The carp herpesvirus is expected to initially kill 70-80% of the carp population in river systems. Gradually, it is expected to be less effective as the virus and host come to a state of equilibrium that allows both to survive. At that point there has been a marked initial reduction in the numbers of the target, and the virus then continues to cause a lower level mortality in the target such that the target numbers never recover to their original levels. As such, the carp herpesvirus is not a silver bullet and integrated carp management programs that use other methods to complement the virus will also be required. These include new broad-scale technologies such as ‘daughterless’ technology to create male-only populations, as well as traditional regional methods such as trapping, the commercial collection of carp, and controlling access of carp to breeding grounds.

Removing the Carp – Restoring Our Waterways
It will be important to target the wetlands which contribute the vast majority of juvenile carp to the Murray-Darling Basin. Releases of the virus in these areas just after the spawning season will hit them when they are most vulnerable, thereby helping to prevent successful carp recruitment. Work to investigate a more virulent strain of the virus will help to overcome any future immunity. The release of the carp herpesvirus will also provide an opportunity to simultaneously restore native fish habitats, improve water quality and restore migratory pathways for native fish, to help ensure that native fish thrive once carp are removed. This, in turn, will help ensure that carp numbers don’t recover.

Other important questions to consider

What about ornamental fish bred intentionally, are they likely to be infected eventually?
Goldfish are not affected by this virus. However, koi carp are completely susceptible, and therefore, for people trying to raise koi, normal biosecurity measures will need to be taken. This may mean treating inflowing water with chemicals, heat, or UV to kill any virus that might be present.

Overseas, the ornamental koi industry apply various biosecurity measures (such as raising water temperature of affected fish) to manage the risks with the carp herpesvirus, which can also include treatment and prevention with a live attenuated (modified live) vaccine. This vaccine is not suitable for use in Australia because vaccinated koi might escape into the wild carp population, resulting in wild koi carp immunity to the carp herpesvirus.

As such, other preventative biosecurity measures will need to be applied by koi breeders. Any new fish introduced to an aquarium should be held in quarantine – about two-four weeks for koi (which could be incubating the virus) and any also for other species that may have had contact with carp/koi (they could be inadvertently carrying the virus on their gills or skin even though it will not cause disease in these species).

What about the businesses that rely on carp?
Businesses that harvest carp to use as fertilizer will still be able to do this, as the initial release will mean a lot of dead fish will be available for this purpose. Even as numbers diminish there will still be carp available as the virus alone will not eradicate all the carp, so it’s important that we have complimentary control measures, like harvesting, to keep numbers low.

Answers to these FAQs have been provided in consultation with our partner organisations